This invention relates to reactors for chemical reduction of nitrogen oxide (NOx) emissions in the exhaust gases of automotive engines, particularly diesel and other engines operating with lean air fuel mixtures that produce relatively high emission of NOx, and method of manufacturing same. More particularly, the invention pertains to an improved stacked shape nonthermal plasma reactor and system for use with diesel engines and the like.
In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing nitric oxides (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz.
An outside or ground electrode is formed by a coating of metal in various forms, including tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surfaces of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas.
Unfortunately, the packed bed design with its loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation needed to make it survive would not be cost effective.
A reactor for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/465,073 entitled xe2x80x9cNon-thermal Plasma Exhaust NOx Reactorxe2x80x9d, now U.S. Pat. No. 6,464,945 which is hereby incorporated by reference herein in its entirety. Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming gas passages and separated by the insulating means. Alternate ground and charge carrying electrodes in the insulating means on opposite sides of the cells are disposed close to, but electrically insulated from, the cells by the insulating means. The electrodes may be silver or platinum material coated onto alumina plates and are coated in a pattern that establishes a separation between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leakage. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field.
There remains a need for an improved non-thermal plasma reactor and an improved method of preparing same which lowers overall cost by reducing manufacturing complexity, the number of components and provides design and manufacturing flexibility. There further remains a need for an improved non-thermal plasma reactor and method that provides reduced manufacturing costs and complexity over improved double dielectric barrier xe2x80x9csandwichxe2x80x9d designs comprising dielectric barrier-conductive material-dielectric barrier-exhaust passage repeating arrangements.
The present invention provides a non-thermal plasma reactor and method of preparing same. The non-thermal plasma reactor element is prepared from a formed shape of dielectric material used as a building block for creating the region of the non-thermal plasma reactor wherein plasma is generated. The formed shape defines an internal cell in the plasma reactor having an exhaust passage for flowing exhaust gas to be treated therethrough. In one embodiment, the formed shape comprises a full cell. In an alternate embodiment, the formed shape comprises a half-cell that is assembled together with a second half-cell to form a full cell.
Individual cells are provided with a conductive print disposed thereon to form electrodes and connectors. In a preferred embodiment, the conductive print comprises a continuous grid pattern having a cutout region disposed opposite the terminal connector for reducing potential voltage leaks. The cutout region provides a distance between the connector and the electrode of adjacent cells sufficient to prevent arc over without diminishing performance. In yet another preferred embodiment, the conductive print is extended over the edge of the cell to provide a site for electrical connection along the side of each cell.
Multiple formed cells are stacked and connected together to form the present multi-cell stack. The upper, outermost cell in the stack is provided with a conductive print on the top and bottom walls. The remainder of the cells in the multi-cell stack have conductive print disposed only on one wall. In a preferred embodiment, the cells are connected with glass glue. Outer plates are provided to insulate the conductive print from the non-thermal plasma reactor housing and to generally protect the conductive print.
The present invention also provides a simple, low cost method for preparing a non-thermal plasma reactor comprising forming, preferably by extruding, a plurality of building block shapes for processing into cells, printing a conductive print onto individual formed cells, assembling the individual cells into a multi-cell stack, preparing electrical connections, applying insulation; and inserting the assembly into the non-thermal plasma reactor housing.
In the half-cell embodiment, the method preferably comprises printing an additional adhesive onto the rails of one of the building blocks for each cell. In a preferred embodiment, the half-cell embodiment employs roll compaction fabrication. In yet another preferred embodiment, the method comprises disposing a catalytic coating on one or both faces of the half-cells.
The present non-thermal plasma reactor is particularly useful for reducing NO, NOx, and particulate constituents in automotive applications. The present reactor and method of preparing same provides the advantages of low cost and durability compared to currently available wire, tubular, or stacked plate designs. The simplified design reduces manufacturing complexity as well as number of components, therefore reducing overall cost. By eliminating the need for spacers between individual cells, the present design and method thus further reduces the total number of components and material cost. In the full cell embodiment, the number of stack components in the stack is reduced by about 80% over prior designs using spacers. In the half-cell embodiment, the number of stack components in the stack is reduced by about 57% over prior designs using spacers.
The present method using formed shapes as building blocks provides flexibility and may be used in conjunction with conventional processing methods. The printing sequence is defined from the top of the multi-cell stack to the bottom, further minimizing the number of components. Use of three-dimensional conductive print simplifies the preparation by eliminating the need for a secondary conductive print along the edge of the multi-cell stack after assembly.
The present invention further provides a non-thermal plasma reactor and method comprising a single structural dielectric barrier. The reactor element includes cells defined by a single structural dielectric barrier comprising a xe2x80x9cconductor-single structural dielectric barrier-exhaust passage-conductorxe2x80x9d arrangement, wherein individual cells of the reactor element are defined by a single structural dielectric barrier rather than a double dielectric barrier.
The conductor may comprise conductive coatings disposed on the single structural dielectric barrier, conductive plates or conductive tubes. Conductors disposed on each side of the exhaust passage are connected to power and ground. When the reactor element is powered with high voltage alternating current, a non-thermal plasma is formed in the exhaust passage for treating constituents present in the exhaust stream passing through the exhaust passage.
The single structural dielectric barrier NTP reactors herein may include cells having any desired shape, including, planar shapes, cylindrical and swept shapes. Examples of suitable single structural dielectric barrier configurations include, but are not limited to, plates, half-box shapes, C-shapes, and tubes.
Advantageously, the present single structural dielectric barrier NTP reactors provide lower manufacturing and material costs over NTP reactors employing double dielectric barrier designs. The present single structural dielectric barrier NTP reactors provide simplified fabrication and reduced material costs due to the elimination of a dielectric barrier for each cell of the NTP reactor element. For example, a C-shaped (planar) single structural dielectric barrier NTP reactor having an element including C-shaped cells uses 50% fewer C-shapes than a comparable double dielectric barrier NTP reactor.
Advantageously, the present single structural dielectric barrier NTP reactors provide similar performance to higher cost double dielectric barrier NTP reactors. At space velocities less than 50,000 inverse hours, single structural dielectric barrier NTP reactors provide equivalent conversion efficiency compared to double dielectric barrier NTP reactors.
Advantageously, the present planar and cylindrical single structural dielectric barrier NTP reactors are prepared using a simplified attachment method that further reduces manufacturing complexity and assembly costs.
In one embodiment, the present single structural dielectric barrier NTP reactors advantageously provide end supports that enable elimination of performance limiting ligaments within the active area of the NTP reactor.